Change Opactiy and Diffuse colour. The Shader doesn't have a Diffuse strength parameter and with the built in Glass Shader you will need raytraced shadows and a Max Ray Trace Depth (Advanced Render Settings) of at least 5 I think from memory. Also this shader has a heavy Index of Refraction setting, depending what type of glass you want Google IOR of glass. Again from memory glass starts around the 1.50 mark and upward depending what type of glass it is.